Heterojunction solar cell based on epitaxial crystalline-silicon thin film on metallurgical silicon substrate design

ABSTRACT

One embodiment of the present invention provides a heterojunction solar cell. The solar cell includes a metallurgical-grade Si (MG-Si) substrate, a layer of heavily doped crystalline-Si situated above the MG-Si substrate, a layer of lightly doped crystalline-Si situated above the heavily doped crystalline-Si layer, a backside ohmic-contact layer situated on the backside of the MG-Si substrate, a passivation layer situated above the heavily doped crystalline-Si layer, a layer of heavily doped amorphous Si (a-Si) situated above the passivation layer, a layer of transparent-conducting-oxide (TCO) situated above the heavily doped a-Si layer, and a front ohmic-contact electrode situated above the TCO layer.

BACKGROUND

1. Field

This disclosure is generally related to solar cells. More specifically,this disclosure is related to a heterojunction solar cell fabricated byepitaxially depositing a crystalline-Si thin film on a metallurgicalgrade Si (MG-Si) substrate.

2. Related Art

The negative environmental impact caused by the use of fossil fuels andtheir rising cost have resulted in a dire need for cleaner, cheaperalternative energy sources. Among different forms of alternative energysources, solar power has been favored for its cleanness and wideavailability.

A solar cell converts light into electricity using the photoelectriceffect. There are several basic solar cell structures, including asingle p-n junction, p-i-n/n-i-p, and multi-junction. A typical singlep-n junction structure includes a p-type doped layer and an n-type dopedlayer. Solar cells with a single p-n junction can be homojunction solarcells or heterojunction solar cells. If both the p-doped and n-dopedlayers are made of similar materials (materials with equal band gaps),the solar cell is called a homojunction solar cell. In contrast, aheterojunction solar cell includes at least two layers of materials ofdifferent bandgaps. A p-i-n/n-i-p structure includes a p-type dopedlayer, an n-type doped layer, and an intrinsic (undoped) semiconductorlayer (the i-layer) sandwiched between the p-layer and the n-layer. Amulti-junction structure includes multiple single-junction structures ofdifferent bandgaps stacked on top of one another.

In a solar cell, light is absorbed near the p-n junction generatingcarriers. The carriers diffuse into the p-n junction and are separatedby the built-in electric field, thus producing an electrical currentacross the device and external circuitry. An important metric indetermining a solar cell's quality is its energy-conversion efficiency,which is defined as the ratio between power converted (from absorbedlight to electrical energy) and power collected when the solar cell isconnected to an electrical circuit.

For homojunction solar cells, minority-carrier recombination at the cellsurface due to the existence of dangling bonds can significantly reducethe solar cell efficiency; thus, a good surface passivation process isneeded. In addition, the relatively thick, heavily doped emitter layer,which is formed by dopant diffusion, can drastically reduce theabsorption of short wavelength light. Comparatively, heterojunctionsolar cells, such as Si heterojunction (SHJ) solar cells, areadvantageous. FIG. 1 presents a diagram illustrating an exemplary SHJsolar cell (prior art). SHJ solar cell 100 includes front electrodes102, an n⁺ amorphous-silicon (n⁺ a-Si) emitter layer 104, an intrinsica-Si layer 106, a p-type doped crystalline-Si substrate 108, and an Albackside electrode 110. Arrows in FIG. 1 indicate incident sunlight.Because there is an inherent bandgap offset between a-Si layer 106 andcrystalline-Si layer 108, a-Si layer 106 can be used to reduce thesurface recombination velocity by creating a barrier for minoritycarriers. The a-Si layer 106 also passivates the surface ofcrystalline-Si layer 108 by repairing the existing Si dangling bonds.Moreover, the thickness of n⁺ a-Si emitter layer 104 can be much thinnercompared to that of a homojunction solar cell. Thus, SHJ solar cells canprovide a higher efficiency with higher open-circuit voltage (V_(oc))and larger short-circuit current (J_(sc)).

Fuhs et al. first reported a hetero-structure based on a-Si andcrystalline-Si that generates photocurrent in 1974 (see W. Fuhs et al.,“Heterojunctions of amorphous silicon & silicon single crystal”, Int.Conf., Tetrahedrally Bonded Amorphous Semiconductors, Yorktown Hts., NY,(1974), pp. 345-350). U.S. Pat. No. 4,496,788 disclosed a heterojunctiontype solar cell based on stacked a-Si and crystalline-Si wafers. Theso-called HIT (heterojunction with intrinsic thin layer) solar cell,which includes an intrinsic a-Si layer interposed between a-Si andcrystalline-Si layers, was disclosed by U.S. Pat. No. 5,213,628.However, all these SHJ solar cells are based on a crystalline-Sisubstrate whose thickness can be between 200 μm and 300 μm. Due to thesoaring cost of Si material, the existence of such a thickcrystalline-Si substrate significantly increases the manufacture cost ofexisting SHJ solar cells. To solve the problem of high cost incurred bycrystalline-Si wafers, this disclosure presents a solution for SHJ solarcells based on epitaxial mono-crystalline Si thin film grown on low-costMG-Si wafers.

SUMMARY

One embodiment of the present invention provides a heterojunction solarcell. The solar cell includes a metallurgical-grade Si (MG-Si)substrate, a layer of heavily doped crystalline-Si situated above theMG-Si substrate, a layer of lightly doped crystalline-Si situated abovethe heavily doped crystalline-Si layer, a backside ohmic-contact layersituated on the backside of the MG-Si substrate, a passivation layersituated above the heavily doped crystalline-Si layer, a layer ofheavily doped amorphous Si (a-Si) situated above the passivation layer,a layer of transparent-conducting-oxide (TCO) situated above the heavilydoped a-Si layer, and a front ohmic-contact electrode situated above theTCO layer.

In a variation on the embodiment, the MG-Si substrate has a purity of atleast 99.9%, and the doping type of the MG-Si is same as the heavilydoped crystalline-Si layer.

In a further variation, the surface of the MG-Si substrate is furtherpurified at a high temperature in an atmosphere of H₂ and HCl.

In a variation on the embodiment, the heavily doped crystalline-Si layeracts as a back-surface-field (BSF) layer. The heavily dopedcrystalline-Si layer is deposited using a chemical-vapor-deposition(CVD) technique. The thickness of the heavily doped crystalline-Si layeris between 1 μm and 10 μm. The doping concentration for the heavilydoped crystalline-Si layer is between 1×10¹⁷/cm³ and 1×10²⁰/cm³.

In a variation on the embodiment, the lightly doped crystalline-Si layeris deposited using a CVD technique. The thickness of the lightly dopedcrystalline-Si layer is between 5 μm and 100 μm, and wherein the dopingconcentration for the lightly doped crystalline-Si layer is between1×10¹⁶/cm³ and 1×10¹⁷/cm³.

In a variation on the embodiment, the thickness of the passivation layeris between 5 nm and 15 nm, and the passivation layer includes at leastone of: undoped a-Si and SiO_(x).

In a variation on the embodiment, the heavily doped a-Si layer isdeposited using a CVD technique. The thickness of the heavily doped a-Silayer is between 10 nm and 50 nm. The doping concentration for theheavily doped a-Si layer is between 1×10¹⁷/cm³ and 1×10²⁰/cm³.

In a variation on the embodiment, the heavily doped and lightly dopedcrystalline-Si layers are p-type doped, and the heavily doped a-Si layeris n-type doped.

In a variation on the embodiment, the heavily doped and lightly dopedcrystalline-Si layers are n-type doped, and the heavily doped a-Si layeris p-type doped

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 presents a diagram illustrating an exemplary SHJ solar cell(prior art).

FIG. 2 presents a diagram illustrating the process of fabricating aheterojunction solar cell in accordance with an embodiment of thepresent invention.

In the figures, like reference numerals refer to the same figureelements.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the embodiments, and is provided in the contextof a particular application and its requirements. Various modificationsto the disclosed embodiments will be readily apparent to those skilledin the art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present disclosure. Thus, the present invention is notlimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the principles and features disclosed herein.

Overview

Embodiments of the present invention provide a SHJ solar cell fabricatedby epitaxially depositing a crystalline-Si thin film on a MG-Sisubstrate. A thin layer of heavily doped crystalline-Si is deposited onthe surface of the MG-Si substrate forming a back-surface-field (BSF)layer, and a thin layer of lightly doped crystalline-Si is deposited ontop of the heavily doped crystalline-Si layer to form a base layer. Anintrinsic thin layer of a-Si is deposited on the base layer to work as apassivation layer. The emitter layer is formed by depositing a heavilydoped a-Si layer. Before the formation of front metal electrodes,transparent-conducting-oxide (TCO) is deposited to form front electrodeand an anti-reflective layer.

Fabrication Process

FIG. 2 presents a diagram illustrating the process of fabricating aheterojunction solar cell in accordance with an embodiment of thepresent invention.

In operation 2A, a MG-Si substrate 200 is prepared. Because MG-Si ismuch cheaper than solar grade or semiconductor grade crystalline-Si,solar cells based on MG-Si substrates have a significantly lowermanufacture cost. The purity of MG-Si is usually between 98% and 99.99%.To ensure a high efficiency of the subsequently fabricated solar cell,the starting MG-Si substrate ideally has a purity of 99.9% or better. Inaddition, the surface of the MG-Si substrate needs to be furtherpurified. In one embodiment, MG-Si substrate 200 is baked at atemperature between 1100° C. and 1250° C. in a chemical-vapor-deposition(CVD) chamber filled with hydrogen (H₂) in order to remove nativesilicon-oxide in the substrate. Afterwards, at approximately the sametemperature, hydrogen chloride (HCl) gas is introduced inside the CVDchamber to leach out any residual metal impurities from MG-Si substrate200, thus further preventing the impurities from diffusing into thesubsequently grown crystalline-Si thin films. Due to the fact that metalimpurities, such as iron, have a high diffusion coefficient at thistemperature, the metal impurities tend to migrate to the surface ofsubstrate 200, and react with the HCl gas to form volatile chloridecompounds. The volatile chloride compounds can be effectively purgedfrom the chamber using a purge gas, such as H₂. Note that themetal-impurity leaching process can be carried out either in the CVDchamber, which is subsequently used for the growth of crystalline-Sithin films, or in another stand-alone furnace. The metal-impurityleaching process can take between 1 minute and 120 minutes. MG-Sisubstrate 200 can be either p-type doped or n-type doped. In oneembodiment, MG-Si substrate is p-type doped.

In operation 2B, a thin layer of heavily doped (doping concentrationgreater than 1×10¹⁷/cm³) crystalline-Si thin film 202 is epitaxiallygrown on the surface of MG-Si substrate 200. Various methods can be usedto epitaxially grow crystalline-Si thin film 202 on MG-Si substrate 200.In one embodiment, crystalline-Si thin film 202 is grown using a thermalCVD process. Various types of Si compounds, such as SiH₄, SiH₂Cl₂, andSiHCl₃, can be used as a precursor in the CVD process to formcrystalline-Si thin film 202. In one embodiment, SiHCl₃ (TCS) is useddue to its abundance and low cost. Crystalline-Si thin film 202 can beeither p-type doped or n-type doped. In one embodiment, boron is addedto make thin film 202 p-type doped. The doping concentration of thinfilm 202 can be between 1×10¹⁷/cm³ and 1×10²⁰/cm³, and the thickness ofthin film 202 can be between 1 μm and 10 μm. The doping level should notexceed the limit, which may cause misfit dislocations in the film.Crystalline-Si thin film 202 is heavily doped to act as back-surfacefield (BSF) and barrier for reducing electron-hole recombination at thesurface of the subsequently grown base film.

In operation 2C, a layer of lightly doped (doping concentration lessthan 1×10¹⁷/cm³) crystalline-Si base film 204 is epitaxially grown ontop of thin film 202. The growing process of base film 204 can besimilar to that used for thin film 202. Similarly, base film 204 can beeither p-type doped or n-type doped. In one embodiment, base film 204 islightly doped with a p-type dopant, such as boron. The dopingconcentration of base film 204 can be between 1×10¹⁶/cm³ and 1×10¹⁷/cm³,and the thickness of base film 204 can be between 5 μm and 100 μm. Notethat compared with a conventional SHJ solar cell that uses acrystalline-Si wafer as a base layer, embodiments of the presentinvention use an epitaxially deposited crystalline-Si film as a baselayer, which can be much thinner than a crystalline-Si wafer. As aresult, the manufacture cost of SHJ solar cells can be significantlyreduced. After deposition, the surface of base film 204 can be texturedto maximize light absorption inside the solar cell, thus furtherenhancing efficiency. The surface texturing can be performed usingvarious etching techniques including dry plasma etching and wet etching.The etchants used in the dry plasma etching include, but are not limitedto: SF₆, F₂, and NF₃. The wet etching etchant can be an alkalinesolution. The shapes of the surface texture can be pyramids or invertedpyramids, which are randomly or regularly distributed on the surface ofbase film 204.

In operation 2D, a backside electrode 206 is formed on the backside ofMG-Si substrate 200. In one embodiment, electrode 206 is formed by firstcoating a layer of aluminum paste on the backside of MG-Si substrate 200and then firing it at a temperature of above 500° C. to form ohmiccontact between electrode 206 and substrate 200.

In operation 2E, a passivation layer 208 is deposited on top of basefilm 204. Passivation layer 208 can significantly reduce the density ofsurface carrier recombination, thus increasing the solar cellefficiency. Passivation layer 208 can be formed using differentmaterials such as intrinsic a-Si or silicon-oxide (SiO_(x)). Techniquesused for forming passivation layer 208 include, but are not limited to:PECVD, sputtering, and electron beam (e-beam) evaporation. The thicknessof passivation layer 208 can be between 5 nm and 15 nm.

In operation 2F, a heavily doped a-Si layer is deposited on passivationlayer 208 to form an emitter layer 210. Depending on the doping type ofbase film 204, emitter layer 210 can be either n-type doped or p-typedoped. In one embodiment, emitter layer 210 is heavily doped with ann-type dopant. The doping concentration of emitter layer 210 can bebetween 1×10¹⁷/cm³ and 1×10²⁰/cm³. The thickness of emitter layer 210can be between 10 nm and 50 nm. Techniques used for depositing emitterlayer 210 include PECVD. Because the thickness of emitter layer 210 canbe much smaller compared with that of the emitter layer in ahomojunction solar cell, the absorption of short wavelength light issignificantly reduced, thus leading to a higher solar cell efficiency.

In operation 2G, a layer of transparent-conducting-oxide (TCO) isdeposited on top of emitter layer 210 to form an anti-reflective layer212. Examples of TCO include, but are not limited to: indium-tin-oxide(ITO), aluminum doped zinc-oxide (ZnO:Al), or Ga doped zinc-oxide(ZnO:Ga). Techniques used for forming anti-reflective layer 212 include,but are not limited to: PECVD, sputtering, and e-beam evaporation.

In operation 2H, metal front electrodes 214 are formed on top ofanti-reflective layer 212. Front metal electrodes 214 can be formedusing various metal deposition techniques at a low temperature of lessthan 400° C. In one embodiment, front electrodes 214 are formed byscreen printing Ag paste. After the formation of front electrodes 214,various techniques such as laser scribing can be used for cell isolationto enable series interconnection of solar cells.

The foregoing descriptions of various embodiments have been presentedonly for purposes of illustration and description. They are not intendedto be exhaustive or to limit the present invention to the formsdisclosed. Accordingly, many modifications and variations will beapparent to practitioners skilled in the art. Additionally, the abovedisclosure is not intended to limit the present invention.

1. A method for fabricating a heterojunction solar cell, comprising:depositing a layer of heavily doped crystalline-Si on the surface of ametallurgical-grade silicon (MG-Si) substrate; depositing a layer oflightly doped crystalline-Si; forming a backside ohmic-contact layer;depositing a passivation layer; depositing a layer of heavily dopedamorphous Si (a-Si); depositing a layer of transparent-conducting-oxide(TCO); and forming a front ohmic-contact electrode.
 2. The method ofclaim 1, wherein the MG-Si substrate has a purity of at least 99.9%, andwherein the doping type of the MG-Si substrate is the same as that ofthe heavily doped crystalline-Si layer.
 3. The method of claim 2,wherein the surface of the MG-Si substrate is further purified at a hightemperature in an atmosphere of H₂ and HCl.
 4. The method of claim 1,wherein the heavily doped crystalline-Si layer acts as aback-surface-field (BSF) layer, wherein the heavily doped crystalline-Silayer is deposited using a chemical-vapor-deposition (CVD) technique,wherein the thickness of the heavily doped crystalline-Si layer isbetween 1 μm and 10 μm, and wherein the doping concentration for theheavily doped crystalline-Si layer is between 1×10¹⁷/cm³ and 1×10²⁰/cm³.5. The method of claim 1, wherein the lightly doped crystalline-Si layeris deposited using a CVD technique, wherein the thickness of the lightlydoped crystalline-Si layer is between 5 μm and 100 μm, and wherein thedoping concentration for the lightly doped crystalline-Si layer isbetween 1×10¹⁶/cm³ and 1×10¹⁷/cm³.
 6. The method of claim 1, wherein thethickness of the passivation layer is between 5 nm and 15 nm, andwherein the passivation layer includes at least one of: undoped a-Si orSiO_(x).
 7. The method of claim 1, wherein the heavily doped a-Si layeris deposited using a CVD technique, wherein the thickness of the heavilydoped a-Si layer is between 10 nm and 50 nm, and wherein the dopingconcentration for the heavily doped a-Si layer is between 1×10¹⁷/cm³ and1×10²⁰/cm³.
 8. The method of claim 1, wherein the heavily doped andlightly doped crystalline-Si layers are p-type doped, and wherein theheavily doped a-Si layer is n-type doped.
 9. The method of claim 1,wherein the heavily doped and lightly doped crystalline-Si layers aren-type doped, and wherein the heavily doped a-Si layer is p-type doped.10. A heterojunction solar cell, comprising: a metallurgical-grade Si(MG-Si) substrate; a layer of heavily doped crystalline-Si situatedabove the MG-Si substrate; a layer of lightly doped crystalline-Sisituated above the heavily doped crystalline-Si layer; a backsideohmic-contact layer situated on the backside of the MG-Si substrate; apassivation layer situated above the heavily doped crystalline-Si layer;a layer of heavily doped amorphous Si (a-Si) situated above thepassivation layer; a layer of transparent-conducting-oxide (TCO)situated above the heavily doped a-Si layer; and a front ohmic-contactelectrode situated above the TCO layer.
 11. The heterojunction solarcell of claim 10, wherein the MG-Si substrate has a purity of at least99.9%, and wherein the doping type of the MG-Si substrate is the same asthat of the heavily doped crystalline-Si layer.
 12. The heterojunctionsolar cell of claim 11, wherein the surface of the MG-Si substrate isfurther purified at a high temperature in an atmosphere of H₂ and HCl.13. The heterojunction solar cell of claim 10, wherein the heavily dopedcrystalline-Si layer acts as a back-surface-field (BSF) layer, whereinthe heavily doped crystalline-Si layer is deposited using achemical-vapor-deposition (CVD) technique, wherein the thickness of theheavily doped crystalline-Si layer is between 1 μm and 10 μm, andwherein the doping concentration for the heavily doped crystalline-Silayer is between 1×10¹⁷/cm³ and 1×10²⁰/cm³.
 14. The heterojunction solarcell of claim 10, wherein the lightly doped crystalline-Si layer isdeposited using a CVD technique, wherein the thickness of the lightlydoped crystalline-Si layer is between 5 μm and 100 μm, and wherein thedoping concentration for the lightly doped crystalline-Si layer isbetween 1×10¹⁶/cm³ and 1×10¹⁷/cm³.
 15. The heterojunction solar cell ofclaim 10, wherein the thickness of the passivation layer is between 5 nmand 15 nm, and wherein the passivation layer includes at least one of:undoped a-Si and SiO_(x).
 16. The heterojunction solar cell of claim 10,wherein the heavily doped a-Si layer is deposited using a CVD technique,wherein the thickness of the heavily doped a-Si layer is between 10 nmand 50 nm, and wherein the doping concentration for the heavily dopeda-Si layer is between 1×10¹⁷/cm³ and 1×10²⁰/cm³.
 17. The heterojunctionsolar cell of claim 10, wherein the heavily doped and lightly dopedcrystalline-Si layers are p-type doped, and wherein the heavily dopeda-Si layer is n-type doped.
 18. The heterojunction solar cell of claim10, wherein the heavily doped and lightly doped crystalline-Si layersare n-type doped, and wherein the heavily doped a-Si layer is p-typedoped.